Catalytic combustors are beginning to be used in recuperated microturbine power generation systems. A microturbine power generation system derives mechanical power for driving the electrical generator from a small gas turbine engine generally known as a microturbine. The engine generally includes at least one turbine that receives the hot combustion gases from a combustor and expands the hot gases to rotate the turbine. The turbine drives at least one compressor wheel that rotates within a compressor housing and supports blades that compress the working fluid passing through. In a recuperated gas turbine engine in which the combustor is a catalytic combustor, typically a gaseous mixture of air and fuel is supplied to the compressor as the working fluid, or separate air and fuel flows are fed to the compressor and the air and fuel subsequently mix after compression. The compressor thus compresses the air and gaseous fuel, which is subsequently fed as an air-fuel mixture to the catalytic combustor, where the mixture is combusted. This arrangement allows for the elimination of a separate gas fuel compressor.
Unfortunately, compressing the fuel in the engine's compressor also gives rise to a likelihood of fuel leaking into the surrounding environment. In any compressor, once the working fluid is raised to a higher pressure, the fluid will seek to flow toward lower-pressure regions by any possible paths, which include leakage pathways that ultimately lead into the ambient air surrounding the engine. Such leakage pathways generally exist, for example, at interfaces between stationary and rotating parts of the compressor. For instance, in a radial compressor as commonly used in microturbines, a leakage pathway exists between the compressor wheel and the stationary compressor housing; this pathway leads into the bearing casing of the compressor. To reduce the amount of leakage into the bearing casing, it is common to include one or more seals between the rotating compressor wheel and the stationary housing. The seals have a high hydraulic resistance and hence discourage fluid from flowing past them into the bearing casing. Typically, the seals comprise labyrinth seals.
The amount of leakage from a compressor typically is relatively small, for example, less than one percent of the total mass flow through the compressor, and thus does not substantially impair the efficiency. Furthermore, the leakage effects are not all negative. For example, one positive effect of the leakage is to prevent oil migrating from the bearing casing into the main gas flow path of the compressor.
However, in the case where the working fluid of the compressor is air and gaseous fuel, even a very small amount of leakage will result in the release of unburned hydrocarbons into the surrounding environment. In a typical recuperated microturbine with a catalytic combustor burning natural gas (e.g., see U.S. Pat. Nos. 4,754,607 and 6,141,953), the air/fuel ratio is close to 100 to 1, meaning that the fuel concentration in the working fluid of the compressor would be close to 14,000 parts per million by volume (ppmvd). If only 0.5% leakage occurs through the compressor seals, then the unburned hydrocarbon emission from this source alone would be 70 ppmvd, which exceeds the acceptable regulatory limits for many areas. In some areas, such as the South Coast Air Quality Management District of the United States, and the Air Quality District for Tokyo and Yokohama, Japan, the maximum acceptable limit may be as low as 10 ppmvd.
Thus, there is a need for a compressor sealing arrangement that can ensure that virtually no fuel escapes into the surrounding environment.